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Getting Started With I2C

Warning:

Revision 2.0 of the Raspberry Pi swaps the connections to I2C buses 0 and 1.

With a revision 2.0 board, if you connect an I2C device to the appropriate header, you will see it when you run i2cdetect 1 instead of i2cdetect 0.

The library now auto-detects whether you are running version 1.0 or 2.0 of the board, so the same code will work on either.

The example:

In this example, we're going to write a program that reads the state of the GPIO pins of an MCP23008 port expander connected to the Raspberry Pi's I2C bus.

Before You Start Coding...

By default, i2c is disabled in the raspbian kernel. To enable it, and check out your installation, follow these instructions

Ensure you are in the i2c group. Run the groups command to list your group membership. For example:

$ groups
nat fuse i2c gpio

You can add yourself to the i2c group with the command:

sudo adduser $USER i2c

You must then log out and in again for Linux to apply the change in group membership.

Check the MCP23008 is connected to your I2C bus and its address is configured as expected. We can see the device on the bus by running the i2cdetect command. Remember to replace 0 with 1 if you hav a revision 2 board.

$ i2cdetect 0
WARNING! This program can confuse your I2C bus, cause data loss and worse!
I will probe file /dev/i2c-0.
I will probe address range 0x03-0x77.
Continue? [Y/n] Y
     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:          -- -- -- -- -- -- -- -- -- -- -- -- -- 
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
20: 20 -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
70: -- -- -- -- -- -- -- --                         

The default address of the MCP23008 is 0x20, but it can be changed (read the chip's data sheet for information about that). If the chip appears at a different address, change the value of the address variable in the code below.

Now Let's Write Some Code!

To use the Quick2Wire I2C API we must import the quick2wire.i2c module. We'll import it with a shorter name for convenience:

import quick2wire.i2c as i2c

Let's define variables to represent attributes of the MCP23008:

address = 0x20
iodir_register = 0x00
gpio_register = 0x09

To communicate with the chip we need to create an I2CMaster object. The I2CMaster class supports the context manager protocol, meaning we can use the with statement to automatically close the bus when the user quits our program by pressing Control-C.

with i2c.I2CMaster() as bus:
    ...

Now we can communicate with the chip. First we'll set all the GPIO pins be inputs by writing to the chip's IODIR register. Setting a bit in the register to 1 switches the corresponding pin to be an input, so setting the byte to 255 (or 0xFF in hex) switches all pins to input. To write to the register we perform an I2C transaction containing a single write operation that writes two bytes: the register to write to and the value of the register.

    bus.transaction(
        i2c.writing_bytes(address, iodir_register, 0xFF))

Then we'll read the value of the chip's GPIO register by performing a transaction containing two operations: a write operation that tells the chip which register we want to read, and a read operation that reads a single byte from that register.

    read_results = bus.transaction(
        i2c.writing_bytes(address, gpio_register),
        i2c.reading(address, 1))

The I2CMaster' transaction method returns a list of byte sequences, one for each read operation performed. Each result is an array of bytes read from the device. So the state of the GPIO pins is the first and only byte of the first and only byte sequence returned.

    gpio_state = read_results[0][0]

We finally print that in hexadecimal:

    print("%02x" % gpio_state)

Putting it all together:

#!/usr/bin/env python3

import quick2wire.i2c as i2c

address = 0x20
iodir_register = 0x00
gpio_register = 0x09

with i2c.I2CMaster() as bus:    
    bus.transaction(
        i2c.writing_bytes(address, iodir_register, 0xFF))

    read_results = bus.transaction(
        i2c.writing_bytes(address, gpio_register),
        i2c.reading(address, 1))

    gpio_state = read_results[0][0]

    print("%02x" % gpio_state)
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